In some known hydraulic braking systems it is desirable for each master cylinder to be of the so called "quick-fill type", for example as disclosed in GB-A No. 2 074 675, so that, on actuation, a relatively large volume of fluid is applied to a respective brake with the result that braking clearances are rapidly taken up. Specifically the volume of a quick-fill chamber at relatively low pressure decreases as fluid is transferred to the brake through a pressure space at relatively high pressure, past a seal on the piston of the master cylinder, until a threshold value is attained at which a valve assembly is operative to place the quick-fill chamber in communication with a reservoir for fluid.
In known master cylinder assemblies of the kind set forth, for example as disclosed in GB-A No. 2 084 678, the transfer passage provides direct communication between the pressure spaces of the master cylinders, and communication between the pressure spaces is cut-off by the transfer valve means when a master cylinder is operated on its own. Connecting the two pressure spaces may mean that the effective length of the master cylinder assembly has to be increased to accommodate the transfer connection and the transfer valve means. When each master cylinder is of the quick-fill type, failure of the transfer connection will lead to failure of the quick-fill chambers and the pressure spaces when both master cylinders are actuated simultaneously and, depending upon the construction of the transfer valve means, may lead to failure of the quick-fill chamber and the pressure space of a single master cylinder, when that master cylinder is operated on its own.